SI Problem Sessions T 3-4 W 12-2 Th 5-6:30 Regener 111 Correction to SI Schedule: Why are you cold getting out of a swimming pool when there is a light breeze? a)Moisture on your skin has good thermal conductivity b)Water has a high heat capacity c)This is a purely psychological effect from sensing wetness and airflow d)The water on your skin is colder than the surrounding air e)It takes 540 calories to evaporate a gram of water; this heat comes from your body

The way to determine pressure using the kinetic-molecular model of a gas is: a)Calculate the kinetic energy and then use the principle of virtual work b)Recognize that there is no heat flow in an isolated system c)Determine the acceleration of each molecule and use F=ma d)Determine how many molecules strike the wall per second, and use the change in momentum of each, with F = dP/dt e)Recognize that if the gas causes the wall to move a distance dx, the work is F dx and this is equal to the loss in kinetic energy of the gas molecules

For a monatomic ideal gas, we can show that the product pV = 2/3 of the translational kinetic energy of the molecules, K tr ! So nRT = (2/3)K tr Temperature is proportional to the kinetic energy per molecule.

20 liters of Argon are in thermal equilibrium with 20 liters of Helium. (These are monatomic gases, M ar =40, M he =4.) Which molecules have more kinetic energy, on average? A] Argon B] Helium C] Both have the same Which molecules are moving faster, on average?

Late Homework: Lose 1/3 per day. BUT… never worth less than 1/3. Do the homework ON TIME, but if you forget or absolutely can’t, DO IT ANYWAY. The kinetic energy of n moles of gas depends ONLY on temperature.

Heat = energy. If we add heat to an ideal gas (held at constant V), it gets warmer. The energy we add goes into increased kinetic energy of the molecules. What is the “heat capacity” (specific heat, but per mole) for a monatomic ideal gas? Also, Q = nC v  T So the “molar heat capacity at constant volume” is

Equipartition of Energy The kinetic energy associated with x motion of the gas molecules is or per molecule The kinetic energy associated with y motion is The kinetic energy associated with z motion is The kinetic energy associated with spring stretching (in a diatomic) is The kinetic energy associated with relative motion along the bond direction (in a diatomic) is

The effects of quantum mechanics: some motions require a “minimum energy”. Memorize: translation (in a gas) is free, rotation is cheaper than vibration

Quantum effect

Which has more internal energy: A mole of N 2 at 100°C or a mole of He at 100°C? A] Nitrogen B] Both have the same energy since T is same C] Helium

Which has more internal energy, 1 liter of N 2 at 1 atm Or 1/2 liter of N 2 at 2 atm? (each with 1 mole of gas) A] 1 liter at 1 atm B] both have the same internal energy C] 1/2 liter at 2 atm

W= Work Done BY a Gas = Memorize!

When gas does work, it loses internal energy (unless energy is added, via heat.) When it does negative work, it gains internal energy

A gas in a piston is taken from state 1 to state 2. The outside pressure is higher than the pressure in the cylinder. For which path does the gas do the largest positive work? For which path does the gas do the most negative work? (I.e. for which path is the most work done ON the gas) Or choose E] no path does work of this sign

In the isobaric process shown, W is: A] + B] - C] 0 D] cannot determine In the isobaric process shown,  U is: In the isobaric process shown, Q is:

In the isochoric process shown, W is: A] + B] - C] 0 D] cannot determine In the isochoric process shown,  U is: In the isochoric process shown, Q is:

In the isothermal process shown, W is: A] + B] - C] 0 D] cannot determine In the isothermal process shown,  U is: In the isothermal process shown, Q is: Let’s do this quantitatively.